Sunday, 29 July 2012

The Pleistocene is the time of the Ice Ages, when great ice sheets rolled across much of the northern hemisphere. Nothing much lived on the ice sheets themselves, just as there is very little today in the heart of Greenland. But, as we've seen, not only were their wide bands of tundra and pine forest reaching across much of today's 'western' world (and, of course, a fair chunk of the Orient), but the ice ages weren't continuous; there were many warm gaps between them.

The mammals of the Pleistocene include what are surely the most familiar fossil mammals to most people, the ones we generally think of when we think of 'after the dinosaurs'. For this was the time of the mammoths and sabretooths. They're familiar to us because, aside from the tiny sliver of warm weather we currently live in, the Pleistocene is the most recent, and therefore the best preserved and the most easily analysed, of all the epochs of the Age of Mammals. It's been the setting for a number of films, books, and TV series - to name just two, the Ice Age cartoons, and the Earth's Children series.

Yet, when we think seriously about Pleistocene animals, there are a couple of important points to bear in mind.

Sunday, 15 July 2012

Black-and-gold howler monkeys. The black one is male, the gold, female.

I've mentioned previously how animals have to juggle their time between different activities, such as feeding, watching for predators, and reproducing, and how that can be affected by things such as the size of the herd they live in (if any). Unsurprisingly, this works both ways, with the size of a herd being affected by the animal's lifestyle.

One of the main reasons for living in a herd is that it makes it easier to watch out for things that are going to try and eat you - you don't all have to be watching all the time, which means that each individual can spend more time eating, or doing whatever else it wants. So a herd has to be large enough for that to be worthwhile, and there may also be a minimum size on viable herds based on such things as the way the animal breeds. On the other hand, a really large herd is going to require a lot of food to sustain it, so the amount of food available is likely to put an upper limit on the herd size.

Sunday, 8 July 2012

Most otters are considered to be 'semi-aquatic' animals. That is, they spend most of their time swimming in rivers or lakes, where they catch the great majority of their food, but they return to dens on the bank in order to sleep and raise their young. However, there is one exception: an otter that is fully aquatic, and is, in fact, the only truly aquatic mammal to have feet, rather than flippers - all the others are seals, whales, dolphins, and the like. This is the sea otter (Enhydra lutris).

Most other otter species will enter salt water on occasion, especially if they live on small islands where fresh water is scarce. The marine otter is unusual in that it habitually hunts at sea, and only occasionally enters rivers, but it still creates dens along the shoreline, and so is considered only semi-aquatic. Generally when otters do swim in the sea, they avoid water that's any deeper than most rivers, and even marine otters won't venture more than about 150 metres (500 feet) offshore. Not so the sea otter.

Sea otters live along the coasts of the northern Pacific. In the south, they reach as far as California, and once even reached the west coast of Mexico, but the majority are now found off the southern coast of Alaska, and there are also sea otters along the coasts of far eastern Siberia and the extreme north of Japan. They are not close relatives of the other American otters (Lontra spp.), but are instead later arrivals on that continent, closer in origin to the various Old World species.

Sunday, 1 July 2012

The "Age of Mammals" is the informal name for the Cenozoic era, the 65 million year slice of Earth's history from the extinction of the dinosaurs to the present day. It is so named because mammals have been the dominant large, land-dwelling animals throughout the era.

Of course, "large, land-dwelling" is something of an arbitrary qualification, and one more rooted in the natural prejudices of our own species than in an actual reflection of Earth's biodiversity. The most numerous animals throughout the era, and, for that matter, through the Age of Reptiles that preceded it, would have been insects. But, unless you're standing in the middle of a swarm of midges, most people don't notice insects in the same way they would notice, say, a herd of antelope, or a prowling tiger. Mammals aren't even the most numerous vertebrates today, and, by sheer species count, it's the fish that are dominant, and some of those are pretty big.

Even on land, in terms of number of species, mammals are the least numerous of the four vertebrate classes - birds come in first, and reptiles still hold on to second place, followed by amphibians. Of course, most of those reptiles are small lizards, and birds are also generally quite small. Even so, there are ostriches, crocodiles, and anacondas, among others, and, in fairness, most mammal species are mouse-sized. So there's a reasonable case that what this should really be is the Age of Birds. But I, for one, am going to stick with the standard term.